Monolithic displays are used predominantly in laptop and portable computers of diagonal sizes of up to 12 inches. They have recently been introduced in desktop personal computers, as well as small televisions. In these applications, a frame surrounds the edge of the glass panels containing the active display elements, i.e., active-matrix liquid-crystal displays (AMLCDs).
Seals for liquid-crystal displays are located at the perimeter thereof and are covered by the frame and housing. Such seals provide a mechanical joint between the top and bottom glass plates of the FPD, as well as contain the liquid-crystal material between the plates. Although the seals are usually polymeric adhesives, a small quantity of glass (or alternative material) spacers contained therein maintains a desired separation distance between the top and bottom plates. Characteristically, this distance between plates ranges from a few microns to 15 microns or more. Typical widths of the seals themselves are customarily a few millimeters.
The polymeric adhesives are usually epoxy-based, thus having a solubility for water and a diffusivity that is appreciable enough to permit water permeation over extended periods of time (days to months). The desired size of seals for individual AMLCD tiles may be an order of magnitude narrower than 1 millimeter; however, the seals for AMLCD FPDs have been proven to be reliable only for widths of 1 millimeter or wider. The rate of diffusion increases exponentially with the reciprocal of the width of the seal, as well as in proportion to the seal thickness and the diffusivity constant for the seal material.
The seals perform several functions, as aforementioned. They mechanically hold the top plate and the bottom plate of the AMLCD in register (both vertically and horizontally), while also providing robustness to the optical stack. In fact, along with the glass plates, the adhesive forms the walls of a reservoir to hold the liquid-crystal material, protecting it from the ambient humidity which may degrade or change its optical properties.
Because the glass plates are not uniformly flat, the adhesive seal, along with spacers, provides a sustaining force to maintain the glass plates essentially parallel to each other and uniformly spaced with respect to one another. This spacing is important for uniform electro-optical response, which is determined by a liquid crystal's electric field that is generated by two respective plates, an anode and a cathode, one on each glass plate.
In order to environmentally protect the liquid-crystals, AMLCDs that are made up of a plurality of individual tiles may need unique sealing designs. Seal width is a major contributor to seam width, since there are two seals in a seam width, i.e, one on each tile perimeter. The more narrow the seam, the more efficient the light transmission is for viewing, and the easier it is to mask and modify the optical properties of the seam. In fact, narrow seals and seams in tiled displays allow for conventionally smaller pixel pitch, hence resulting in displays of greater resolution.
However, using narrow, conventional, epoxy seals may degrade both their mechanical and permeation performances. Therefore, tiled displays may need special attention to mechanical design, for the reliability of the individual tiles must be ensured, so as to prevent any effects from humid environments and provide forces maintaining the registration of the top and bottom plates of the AMLCD.
At the intersection of the tiles, the edge dimensions thereof are preferably maintained so that the interpixel spacing remains uniformly periodic throughout the tiles and across the seams, and the pitch minimized from tile to tile, as well as within each tile. Nevertheless, sealing integrity and reliability must be maintained for all of the display's tile edges. The tile perimeter design elements incorporated into the dark, interpixel space include:
a) the two seals on neighboring tiles; PA1 b) the contact pads for the electrical connections, if they are located outside the seal; PA1 c) the electrical connection thickness between the walls of the two tiles; PA1 d) spacing between the tiles to allow for extraction of a tile for reworking, if so desired, without touching its neighboring tile(s); PA1 e) glass-cutting tolerances; and PA1 f) location tolerances.
All of the above-cited components and spaces are within a dark space between tiles, and will set the width of the intertile spacing and the pixel pitch.
The liquid-crystal material in tiled, flat-panel displays has the same susceptibility to moisture as do monolithic, liquid-crystal displays, but the design and manufacturing problems become more thorny. The need for invisible seams between any one tile and its adjacent tiles results in the need to keep all dimensions in this area, including the width of the adhesive seal, extremely small (approximately a few tenths of a millimeter, at most).
Electrical connection to the display is usually achieved by utilizing flexible connections, either from a printed circuit board containing drive and addressing electronics, or with an integrated-function chip carrier/flexible cable design. In either case, such electrical connection decreases the space available for sealing for a given pixel pitch.
U.S. Pat. No. 5,668,569, described the advantage of positioning row and column drivers at a common edge of a tile and to deliver row and column signals to the tile from the common edge. This may eliminate the need for electrical connections at narrow ledges of interior tiles in a FPD of tiles 2.times.N, where N&gt;2. This in turn may improve optical performance of the display.
It is advantageous to provide a reliable, semi-hermetic or hermetic seal without sacrificing the other functional requirements of tiled, flat-panel displays.
It is also advantageous to furnish fabrication techniques that improve the symmetry of the location of impurities in the liquid-crystal material, particularly in filling the space between the top and lower plates of the display or tile.
It is further advantageous to provide a seal that facilitates the electrical interconnection of tiles.
It is further advantageous to provide fabrication techniques that allow testing of tiles to assure quality before committing tiles to further assembly.